The present invention relates to an optical waveguide interferometer composed of planar optical waveguides, and more particularly to a technique that compensates for the polarization sensitivity of the optical interferometer, or on the contrary enhances the polarization dependence, by utilizing the dependence of waveguide birefringence on waveguide core width.
Today, optical wavelength division multiplexing communication systems (WDM systems) utilizing a plurality of optical wavelengths are being developed intensively to increase communication capacity. In the optical wavelength division multiplexing communication systems, arrayed waveguide grating optical wavelength multi/demultiplexers (abbreviated to AWGs from now on) are widely used as optical wavelength multi/demultiplexers for multiplexing a plurality of optical signals with different wavelengths at a transmitting side, and for demultiplexing a plurality of optical signals passing through an optical fiber to different ports at a receiving side.
FIG. 18 shows a circuit configuration of a conventional AWG. Light launched into an input waveguide is diffracted in parallel with a substrate 3 in a first slab optical waveguide 2, and coupled to a plurality of arrayed waveguides 4. Since the adjacent arrayed waveguides 4 have a fixed optical path difference, a plurality of light beams have phase differences depending on the wavelengths when they are coupled to a second slab optical waveguide 5. As a result, the focal points made by the interference between the plurality of light beams change their positions depending on the wavelength. Thus, disposing a plurality of output waveguides 6 at the points of focus in advance makes it possible for the AWG to function as an optical wavelength multi/demultiplexer for multiplexing or demultiplexing the plurality of optical wavelengths in the block.
In the reported AWGs at present, the plurality of arrayed waveguides 4 are designed to have the same core width. The AWGs are fabricated using waveguides of a variety of materials such as glass, polymer and semiconductors, and their results are reported (M. K. Smit, xe2x80x9cNew focusing and dispersive planar component based on an optical phased array,xe2x80x9d Electronics Letters, vol. 24, no. 7, pp. 385-386, March 1988; Y. Hida, et al., xe2x80x9cPolymeric arrayed-waveguide grating multiplexer operating around 1.3 mm,xe2x80x9d Electronics Letters, vol. 30, pp. 959-960, 1994; and M. Zirngibl, et al., xe2x80x9cPolarization compensated waveguide grating router on InP,xe2x80x9d Electronics Letters, vol. 31, no. 19, pp. 1662-1664, 1995).
Generally, an optical waveguide formed on a planar substrate has different effective refractive indices for the TM light with an electric field component vertical to the substrate and for the TE light with an electric field component parallel to the substrate. The difference between the effective refractive indices is called waveguide birefringence, and defined by the following equation (1).
B=nTM=nTExe2x80x83xe2x80x83(1) 
where B is the waveguide birefringence, and nTM and nTE are the effective refractive indices of the TM light and TE light. The waveguide birefringence is caused by stress-induced birefringence, structural birefringence or the like.
AWG center wavelengths of the TM light and TE light are expressed by the following equations (2) and (3).                               λ          TM                =                                                            n                TM                            ·              Δ                        ⁢                          xe2x80x83                        ⁢            L                    m                                    (        2        )                                          λ          TE                =                                                            n                TE                            ·              Δ                        ⁢                          xe2x80x83                        ⁢            L                    m                                    (        3        )            
where xcexTM and xcexTE are the AWG center wavelengths of the TM light and TE light, xcex94L is the length difference between adjacent arrayed waveguides, and m is a diffraction order (integer).
As seen from the foregoing equations (1)-(3), when the waveguide birefringence B is present, the AWG center wavelengths xcexTM and xcexTE of the TM light and TE light differ from each other. Basically, a silica-based glass waveguide has little dependence of a propagation loss on the polarization. However, since the center wavelengths differ for the TM light and TE light, the AWG has a problem of the polarization sensitivity that its characteristic changes depending on the polarization state of incident light.
(First Example of Conventional Technique)
FIG. 19 shows a method of eliminating the polarization dependence. It inserts into the arrayed waveguide 4 a half-wave plate 7 whose principal axis inclines 45xc2x0 at the center of the AWG via a groove 8 (Y. Inoue, et al., xe2x80x9cPolarization sensitivity elimination in silica-based wavelength-division multiplexer using polyimide half waveplate,xe2x80x9d IEEE J. Lightwave Technol., vol. 15, no. 10, pp. 1947-1957, October 1997).
The half-wave plate 7 operates as a polarization mode converter between the TM light and the TE light so that the polarization sensitivity is eliminated by exchanging the TM light and the TE light at the center of the AWG to average the overall characteristic.
(Second Example of Conventional Technique)
Another method of eliminating the polarization sensitivity of the AWG is reported. It reduces the thermal stress in the fabrication process of the AWG by providing the silica-based glass with a thermal expansion coefficient corresponding to the thermal expansion coefficient of the silicon substrate by adding much dopant to the silica-based glass, thereby eliminating the polarization sensitivity (S. Suzuki, et al., xe2x80x9cPolarization-insensitive arrayed-waveguide gratings using dopant-rich silica-based glass with thermal expansion adjusted to Si substrate,xe2x80x9d IEE Electron. Lett., vol. 33, no. 13, pp. 1173-1174, June 1997).
More specifically, adjusting the stress imposed on the silica-based glass layer from the silicon substrate to a value between xe2x88x921 Mpa and 1 Mpa enables the absolute value of the waveguide birefringence to be limited equal to or less than 2xc3x9710xe2x88x925, where the negative sign designates compressive stress and the positive sign designates tensile stress.
The second method of the conventional technique is a more promising candidate than the first method of the conventional technique because it obviates the additional step involved in inserting the half-wave plate 7, and prevents excess loss as well. The second method, however, has a problem of readily causing cracks in the silica-based glass layer during the fabrication process of the AWG because the compressive stress of the glass is very weak. In addition, since the silica-based glass layer contains a lot of dopant, it is poor in long term weather resistance, and brings about crystallization in the waveguide which will increase the optical insertion loss of the waveguide. The low reliability is a critical problem with the optical communication component to be solved urgently.
In summary, the two methods of achieving the polarization-independence described in the conventional techniques have the problems to be solved. The first conventional method using the half-wave plate has a problem of requiring the additional step involved in inserting the half-wave plate, and of bringing about the excess loss of light. On the other hand, the second conventional method of eliminating the thermal stress of the glass by increasing the dopant of the silica-based glass has a problem of its reliability.
The present invention is implemented to solve the foregoing problems. Therefore, an object of the present invention is to provide a low cost, high reliability, polarization-independent optical waveguide interferometer.
We found that the waveguide birefringence varies depending on the core width. Utilizing this phenomenon, the present invention solves the problem of polarization sensitivity of the AWG without additional job or component. More specifically, the polarization sensitivity of the AWG is eliminated by varying the effective core widths of the waveguides of the arrayed waveguide one by one.
To accomplish the object, according to a first aspect of the present invention, there is provided an optical waveguide interferometer composed of optical waveguides on a substrate, wherein the optical interferometer comprises an optical branching section, a plurality of optical waveguides with different lengths, and an optical coupling section, and wherein core widths averaged in a longitudinal direction of the plurality of optical waveguides with different lengths differ from one another.
According to a second aspect of the present invention, there is provided an optical waveguide interferometer composed of optical waveguides on a substrate, wherein the optical interferometer comprises an optical branching section, a plurality of optical waveguides with different lengths, and an optical coupling section, and wherein core widths averaged in a longitudinal direction of the plurality of optical waveguides with different lengths are wider in shorter optical waveguides and narrower in longer optical waveguides.
Here, longitudinal integral values of birefringence of the plurality of optical waveguides with different lengths may be equal to one another among the plurality of optical waveguides.
The optical waveguide interferometer may be an arrayed waveguide grating optical wavelength multi/demultiplexer including two slab waveguides and a plurality of arrayed waveguides with different lengths each other interconnecting said slab waveguides, and core widths of the plurality of arrayed waveguides may differ from one another depending on lengths of the arrayed waveguides, and each core width may be fixed except for connecting sections with the slab waveguides.
The optical waveguide interferometer maybe an arrayed waveguide grating optical wavelength multi/demultiplexer including two slab waveguides and a plurality of arrayed waveguides with different lengths each other interconnecting said slab waveguides, and the plurality of arrayed waveguides may be each composed of waveguides with two-types of core widths except for connecting sections with the slab waveguides, and a ratio between lengths of the waveguides with the two-types may differ from one another among the plurality of arrayed waveguides.
The optical waveguide interferometer may be an arrayed waveguide grating optical wavelength multi/demultiplexer including a first slab waveguide, a second slab waveguide, a plurality of arrayed waveguides that interconnect them and have different length from one another, at least one input waveguide connected to the first slab waveguide, and at least one output waveguide connected to the second slab waveguide, and core widths averaged in a longitudinal direction of the plurality of arrayed waveguides may be wider in shorter optical waveguides and narrower in longer optical waveguides, and as a result of this, longitudinal integral values of birefringence of the plurality of arrayed waveguides may be equal to one another among the plurality of arrayed waveguides.
The plurality of arrayed waveguides may each have a constant core width in their longitudinal directions except for connecting sections with the first and second slab waveguides.
The plurality of arrayed waveguides may be each composed of waveguides with at least two-types of core widths except for connecting sections with the first and second slab waveguides, and a ratio between lengths of the waveguides with the at least two-types may differ from one another among the plurality of arrayed waveguides.
The waveguides with the at least two-types of the core widths may be interconnected by a tapered section that continuously changes its width.
A tapered section interconnecting the waveguides with different core widths may be composed of a plurality of waveguides that have different widths and are connected serially to one another.
The plurality of arrayed waveguides with different lengths from one another may each include a straight line waveguide at their central section.
The optical waveguide interferometer may be a Mach-Zehnder interferometer including two optical couplers, and two optical waveguides that interconnect them and have different lengths, and the two optical waveguides may have core widths different from each other at least in part, and an averaged core width of the shorter waveguide may be wider than an averaged core width of the longer waveguide.
The optical waveguide interferometer may be a Mach-Zehnder interferometer including two optical couplers, and two optical waveguides that interconnect them and have different lengths, and the two optical waveguides may have core widths different from each other at least in part, and longitudinally integral values of birefringence of the two optical waveguides may differ from each other by half optical wavelength used.
The optical waveguide interferometer may be composed of silica-based glass optical waveguides on a silicon substrate.
An internal stress of a core film constituting each optical waveguides of the optical waveguide interferometer may be two times greater than an internal stress of an upper cladding film.